Tales from the front line in the hunt for the perfect SCART television for use with the MAME arcade game emulator.

Tuesday, 23 July 2013

Adjusting vertical linearity

While fiddling with the E3000 chassis recently, I came to reaslise how important vertical linearity is to the attainment of good overall geometry.

I decided to experiment with creating my own test patterns to check the linearity of my Calida 5072. I created a black and white bitmap in MS Paint with 5 equally distributed lines. I used an odd number of lines so that I can get the 5 lines equally spaced. Since I play so many 224 lines games, I created a 225 line mode to use for testing:

225 line linearity test screen

I used Arcade_OSD to access my test modeline and then displayed my test image fullscreen. Next, I used a small ruler to measure the space between each line. Much to my surprise, my even looking lines had up to 15mm difference in the spacing between them!

I got thinking... there had to be a way of getting this accurate. Here's the solution I came up with:

Paper mask with equidistant marker lines

Close-up of the mask

I prepared an A3 paper mask to help me line up the 5 test lines. I had to juggle between V-Amp, V-Pos, V-Lin and V-Sym in the Service Menu. Changing one value will most likely require an adjustment of the other. However, despite the interaction, it only took me around 10 minutes to get this as close to perfect as possible.

After that, I loaded up regular cross hatch and adjusted the horizontal size and centring. Next, I fiddled with the EW and Trap controls. The resulting grid wasn't perfect but it was extremely close for a CRT!

The benefit of all this fiddling is how good the games now look, especially when you have a vertical scroll effect. So even and smooth now!